Cylinder deactivation is one method that may improve fuel economy while maintaining reserve torque capacity for elevated load conditions. During cylinder deactivation, the air charge in operating cylinders increases resulting in higher thermal efficiency and lower pumping losses, thereby decreasing fuel consumption. When higher torque is desired, additional cylinders may be activated to provide additional torque up to the capacity of the engine. Consequently, the ability to activate and deactivate cylinders on-demand may be used to improve fuel economy while retaining torque capacity.
One method to control electromechanical intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 6,332,446. This method provides electromechanical valves in an internal combustion engine operated by a controller. The controller causes at least one of the plurality of cylinders to operate in a cylinder deactivation mode in which the intake stroke, combustion stroke, and exhaust stroke are inhibited, and controls the pressure within each inactive cylinder that operates in the cylinder deactivation mode.
The above-mentioned method controls cylinder valves in the deactivated cylinders in an effort to reduce engine vibration. However, this method can increase temperatures in active cylinders. To achieve equivalent engine torque between cylinder modes, where the cylinder mode determines the number of active cylinders, the flow of air through the engine is roughly the same. For example, where a four cylinder engine is producing X N-m of torque with Y Kg/s of air, the same engine will use roughly Y Kg/s of air to produce X N-m of torque operating in two cylinder mode, provided that two cylinders can produce X N-m of torque. Differences in air flow through the engine can be attributed to engine friction, spark advance, and pumping work. As a consequence, the amount of air entering a single cylinder during cylinder deactivation increases. This can produce an increase in cylinder pressure and result in higher than desirable cylinder and valve temperatures.
Furthermore, the above-mentioned method deactivates cylinders to improve fuel economy, but it is limited to a torque capacity that is based on the number of operating cylinders. For example, where a four-cylinder engine is operating in two-cylinder mode, the engine has roughly one half the total torque capacity of the engine. In this example, each cylinder represents a quarter of the torque capacity of the engine. Therefore, the method is limited to selecting a torque capacity, based on a number of cylinders, which is often in excess of the desired torque.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of cylinder deactivation that can offer improvements.